Optical transmission techniques deployed to date have been mostly based on intensity modulated signals. Amplitude-shift keying (ASK), also known as on-off keying (OOK), is a simple form of modulation that represents the digital data as variations in the amplitude of its carrier wave. In optical transmission, ASK encodes a bit to a logical “one” in the presence of a pulse of light and to a logical “zero” in the absence or near absence of a light pulse.
Optical dual binary (ODB) is another form of modulation, where the optical signal, at the receiving side of the transmission fiber, is directed to a photo diode. The photo diode converts the light energy to an electrical current, which is subsequently decoded into the received bit stream.
As transmission rates increase above 10 Giga bits per second (Gbps), more complex modulation formats are being developed. These modulation formats generally result in superior performance over other modulation formats such as on-off keying.
At transmission rates of 40 Gbps and higher, phase modulation (PM) is a very popular modulation format. Phase modulation represents a signal as variations of phase of its carrier wave. Phase modulation employs an optical carrier, which for wavelength division multiplexing (WDM) systems is in the range of 190 Tera Hertz (THz). This optical carrier is phase modulated to represent the logical “ones” and “zeros” of the bit stream. Most phase modulated formats are differentially encoded, such that the phase difference between any two successive bit intervals represents a logical “one” or a logical “zero” to the receiver.
Differential phase shift keying (DPSK) is another modulation technique that represents the digital data as variations in the phase of its carrier wave. In differential phase shift keying, the receiver decodes a logical “one” bit if the difference between the phase in the current bit interval differs from the phase of the previous bit interval by one hundred and eighty degrees. If the phase between the current and last interval does not change, the receiver decodes the phase modulated optical carrier as a logical “zero.”